1. Field of the Invention
The present invention relates to a tire air pressure estimating apparatus for indirectly estimating tire air pressure or change thereof in an automobile or the like.
2. Related Art
Japanese Patent Application Laid-Open No. 5-133831 discloses an apparatus of estimating an air pressure state of a tire. In the apparatus, a resonance frequency of a tire is extracted by carrying out frequency analysis with respect to a signal (wheel speed signal) including vibration frequency components of a tire. The tire air pressure state is detected based on the extracted resonance frequency.
The resonance frequency used when tire air pressure is estimated is about 30 through 50 Hz. However, as mentioned below, tire air pressure cannot be accurately estimated depending on situations in which the vehicle is put. For example, the tire air pressure can be estimated using a resonance frequency in a range of about 30 through 50 Hz when the running speed of a vehicle is in a low or middle speed range, like when the vehicle runs in an urban district or the like. However, when the running speed of the vehicle reaches a high speed range, a vibration phenomenon of a tire is difficult to cause. As a result, a power spectrum level of the resonance frequency is lowered and accuracy in estimating the tire air pressure is worsened.
For the problems as mentioned above, Japanese Patent Application Laid-Open No. 8-219920 teaches an apparatus in which plural resonance frequencies of a tire are extracted from plural frequency ranges, such as a range of about 30 through 50 Hz and a range of about 60 through 90 Hz, of the vibration frequency components included in the wheel speed signals. That is, when the running speed of a vehicle reaches a high speed range, the frequency range where the resonance phenomenon is intensified is changed from that of when the running speed of the vehicle is in a low through middle speed range. Therefore, when the frequency range used for estimating the tire air pressure is changed in accordance with the vehicle running speed range, the accuracy in estimating tire air pressure can be enhanced.
The inventors of the present application confirmed that a higher-order resonance frequency (for example, in a range of about 60 through 90 Hz) included in the wheel speed signal, greatly changes in response to change of the tire air pressure in a drive wheel of a vehicle, which is a characteristic suitable for estimating the tire air pressure. However, the inventors also found that, even when the higher-order resonance frequency is extracted, the change thereof in response to the change of the tire air pressure is small in a driven wheel, whereby accurate estimation of the tire air pressure is difficult.
Therefore, it is an object of the present invention to provide an apparatus capable of accurately estimating tire air pressures of respective wheels in accordance with characteristics thereof, even when running conditions of the vehicle is changed.
In a tire air pressure estimating apparatus according to the present invention, a vehicle speed detecting device successively calculates wheel speeds of respective wheels when a vehicle is running. An extracting device extracts at least one of a tire resonance frequency and a tire spring constant with respect to each of tires from vibration frequency components included in wheel speed signals which are detection results of the wheel speed detecting device. A first tire air pressure estimating device estimates tire air pressures of drive wheels based on the tire resonance frequencies or the tire spring constants extracted by the extracting device.
The tire air pressure estimating device further comprises a rotational state value calculating device for calculating rotational state values of the tires based on the wheel speed signals detected by the wheel speed detecting device, and a second tire air pressure estimating device for estimating the tire air pressures of driven wheels based on a deviation of the rotational state values calculated by the rotational state value calculating device.
That is, the tire air pressures of the drive wheels are estimated by the first tire air pressure estimating device carrying out frequency analysis in respect of the wheel speed signals, because the tire resonance frequencies or tire spring constants of the drive wheels can be extracted from vibration frequency components of tires included in wheel speed signals without being much influenced by vehicle running conditions such as a vehicle speed or the like. To the contrary, the tire air pressures of the driven wheels are estimated by using a dynamic load radius of a tire. This is because accuracy can not be ensured in estimating the tire air pressure by using frequency analysis in respect of the wheel speed signals, since a power spectrum level of vibration frequency components of a driven wheel tire is greatly changed in accordance with the vehicle running conditions such as the vehicle speed or the like.
In this way, the tire air pressures can be estimated accurately regardless of the vehicle running conditions or whether the wheel is a drive wheel or a driven wheel.
Alternatively, a tire air pressure estimating apparatus according to the present invention may comprise an extracting device for extracting a plurality of tire resonance frequencies or a plurality of tire spring constants from tire vibration frequency components included in a wheel speed signal with respect to each wheel, a selecting device for selecting a tire resonance frequency or tire spring constant used in estimating tire air pressure among the plurality of tire resonance frequencies or tire spring constants in accordance with a running condition of a vehicle, and a determining device for determining based on the running condition of the vehicle whether the wheel speed signals are suitable for estimating the tire air pressure based on the resonance frequency or spring constant extracted therefrom and selected by the selecting device.
The inventors found from their study that the wheel speed signal of a driven wheel includes a higher-order resonance frequency which greatly changes in response to the change of tire air pressure even after a running speed of the vehicle reaches a high speed range, when the vehicle is in one of predetermined running conditions. Therefore, the tire air pressure of the driven wheel can be estimated based on the vibration frequency components containing such a resonance frequency, in the similar manner with the drive wheel.
That is the tire air pressure of a drive wheel is basically estimated all the time based on the resonance frequency or spring constant selected by the selecting device, because the plurality of resonance frequencies or spring constants extracted with respect to the drive wheel is not considerably influenced by the vehicle running conditions such as the vehicle speed or the like.
To the contrary, the tire air pressure of a driven wheel is estimated based on the resonance frequency or spring constant extracted from the wheel speed signal detected when the vehicle is in one of the predetermined running conditions. For example, a condition in which braking force is operated on a vehicle is detected as one of the predetermined running conditions. The tire air pressure of the driven wheel is estimated based on the resonance frequency or spring constant extracted from the wheel speed signal detected during the braking operation of the vehicle.